[server] add a django cleanup command

[yomguy]

This command should remove:

• any singleton Selection
• any result without file
• any result file without result pointing to it

Also advisable:

• a result.pre_delete signal that runs os.remove on any hdf5 or audio file
• define a workflow to remove a result (a waveform for example)

[Tointoin]

* [x]  a result.pre_delete signal that runs `os.remove` on any hdf5 or audio file

There is a bug on sandbox instance @yomguy after dealing with result.pre_delete.

I explained it on issue 180